In recent years, magnetic disk apparatuses have been improved rapidly by decreasing the size and increasing the capacity thereof. The capacity of a magnetic disk apparatus has been increased by increasing the track density of the magnetic disk, and the track pitch will be further reduced in the future. Therefore, in order to record/reproduce data to/from a high-density magnetic disk, it is necessary to quickly and precisely position a magnetic head to the target track among other tracks formed at a small track pitch.
Typically, with a magnetic disk apparatus, servo information, based on which the magnetic head is positioned, is recorded in advance on the magnetic disk at a predetermined angular interval. The magnetic head is positioned based on the servo information. Specifically, the positioning operation is performed at a certain sampling cycle that is determined by the rotational speed of the magnetic disk and the number of servo information recorded per track (the number of servo sectors). The magnetic head reads the servo information at the sampling cycle, whereby a position error signal indicating the positional error of the magnetic head with respect to the target track is produced, and the magnetic head is positioned so that the position error signal is minimized.
In order to precisely position the magnetic head, it is necessary to shorten the sampling cycle, thereby increasing the control frequency of the magnetic head positioning system. However, in order to shorten the sampling cycle, it is necessary that more servo information is recorded on the magnetic disk, which results in a decrease in the data format efficiency. Moreover, the actuator for positioning the head may have a natural vibration mode of higher-order. Therefore, if the control frequency is increased in order to increase the positioning precision, the positioning system may become unstable due to the natural vibration. In practice, since the control band is limited by the natural vibration of the actuator itself, there is a limit to increasing the control frequency of the positioning system.
Moreover, as the size of magnetic disk apparatuses has been decreased in recent years, disturbance such as the bearing friction force that acts upon the actuator or the elastic force of the flexible print circuit (FPC) that connects the actuator with a circuit substrate presents a factor that further deteriorates the positioning precision. In view of this, techniques have been proposed in the art for improving the positioning precision by compensating for such disturbance. For example, Japanese Laid-Open Patent Publication No. 9-231701 proposes a head positioning method, in which a head position signal is obtained from servo information recorded on the magnetic disk, and external forces are compensated for by disturbance estimation means that receives the head position signal and an actuator driving signal. With this technique, the actuator driving signal is obtained by adding together a disturbance compensation signal produced by the disturbance estimation means and a position control signal produced by using a position error signal that indicates the positional error of the magnetic head.
With this head positioning method, however, when positioning the magnetic head at the target track, i.e., when moving the magnetic head at a high velocity toward the target track from another track (seek operation), the position control signal may overshoot, whereby the driving signal itself is unstable. In such a case, the disturbance estimation means, which uses the driving signal as an input, cannot produce an appropriate disturbance compensation signal by accurately estimating the disturbance acting upon the actuator. As a result, it may become difficult to accurately position the magnetic head at the target track, or the magnetic head may take a long time following the target track, thereby lowering the performance of the magnetic disk apparatus.
Note that this is not a problem particular to magnetic disk apparatuses, but is a common problem among disk apparatuses in general.